1. Field of the Invention
The present invention relates to a two-layer structure used as a material for housings possessing an excellent electromagnetic shielding effectiveness and the like. More particularly, the present invention relates to a two-layer structure molded by using thermoplastic resin comprising one layer (a) of thermosplastic resin in which no electroconductive filler is mixed, and the other layer (b) of thermosplastic resin in which electroconductive filler is mixed wherein one or more projections which penetrate said layer (b) are formed on said layer (a).
2. Description of the Prior Arts
Recently, thanks to the remarkable development of electronical arts including the development of the IC, LSI and etc., the digital instruments using an IC, LSI and the like, such as the computer, the electronic game machine, the television game machine, the switching electric power source of the electronic cash register, the digital clock, the desk electric computer, the word processor and the like have widely spread to the world. Said electric articles and electronic instruments advantageously use plastics as materials of their housings because the housings using plastics as the matterials are easily mass-produced, because plastics have an excellent moldability and processability and have good insulating property.
On the other hand, digital instruments generate the electromagnetic wave more than 10,000 pulse/sec to the space and said electromagnetic wave may disturb the satisfactory radio, TV, wireless telegraph reception and the like because plastics as materials of housings are transmissible for the electromagnetic wave and therefore the problem of shielding from the electromagnetic wave has become important to protect against the disturbance of the electromagnetic wave.
Hitherto, the thermoplastic resin mixed with the metal filler has been studied as the material of housings. Nevertheless, said plastics have disadvantages that the metal filler in the plastics may be exposed on the surface of molded housings, so the appearance of housings becomes inferior, and the mechanical strength and the insulation resistance decrease and the like. Formerly, housings having a two-layer structure molded by using thermoplastic resin comprising one layer (a) of thermoplastic resin in which no electroconductive filler is mixed, and the other layer (b) of thermoplastic resin in which electroconductive filler is mixed were provided by inventors of the present invention. Said housings have desirable appearance owing to said layer (a) as the outer layer and a desirable electromagnetic shielding effectiveness owing to said layer (b) as the inner layer.
Nevertheless, the mechanical strength of the layer (b) is somewhat less than that of the layer (a) as electroconductive filler is mixed in thermoplastic resin of the layer (b), and moreover, necessary projection(s), such as boss(es), franges, rib(s) and the like, has(have) been generally formed on the layer (b) having less mechanical strength. Thus, the stress on the two-layer structure may concentrate on said projection(s) and as a result, there has arisen a high risk of fractures of said two-layer structure around the site of the projection(s) of the layer (b).